I'm seeking a script that with one click, can go to an online auction site (eBay, for example), find that users open auctions page listing, and add an image/banner to each of the pages, that have open auctions.

Think membership situation, where a member would want to add the Association logo to each of their open auctions.

I have seen this done, in a couple places, and can't sniff out how it's done, nor do it myself.

This is a quick script that I use to get the auction numbers of the items I'm selling on ebay. It's a start to your script. The rest of your script can be done using more of the LWP mod. It is just automating form submission. You’re just bypassing the log in form and sending the variables need straight to the CGI program. If I have time I will dig up an example for you.